The prior art, as exemplified in U.S. Pat. Nos. 3,938,090, 4,310,720 and 4,430,728, contains a number of security systems for preventing unauthorized access to central or host computers connected to a public telephone system. Previous security systems have employed data encryption schemes, secret access codes, and access limitations and alarms. Such prior art security systems employ either a main computer to analyze the incoming access codes or data or employ a separate security computer which prevents connection of the telephone line to the computer until proper access codes have been received on the telephone line. These prior art types of security systems have been defeated by unauthorized callers employing personal computers as terminals. This sometimes resulted in destruction of data, theft of data, unauthorized fund transfer, etc. With increasing use of computers for banking services, credit rating data, private business information, and the like, there exists a need for a relatively simple and inexpensive security system to prevent computer access by an unauthorized caller.